A gas turbine engine, for example, a turbofan jet engine, includes a fan coupled to a shaft. As the fan rotates, ambient air is drawn into the engine through an inlet thereof. A portion of the drawn air passes through a bypass flow path and escapes through an exhaust port of the engine and creates thrust that propels a vehicle. Another portion of the drawn air is directed through one or more compressors that compress and pressurize the air. The compressed air is directed to a combustor in which the compressed air is combined with a fuel and ignited. Such ignition causes combustion of the fuel and the compressed air, and produces rapidly expanding gasses. The gasses pass through a turbine that includes one or more turbine stages coupled to the shaft, and are exhausted through the exhaust port. The gasses rotate the turbine, which then causes the shaft to rotate. Rotation of the shaft rotates the fan to draw in more ambient air into the inlet port of the engine.
The compressor of the engine may include a combination of axial and centrifugal compressors. The airflow through the axial compressor is generally parallel to the shaft coupled to the axial compressor, and the airflow through the centrifugal compressor is generally perpendicular to the shaft. Both types of compressors include rotatable components that are coupled to the shaft.
The axial compressor includes one or more rotor stages interleaved with one or more stator stages. Each such stator stage includes one or more vanes. In some engines, the pitch of the vanes in each stage relative to the airflow through the axial compressor is varied in accordance with the rotational speed of the rotor stages. In some such engines, the vanes of a particular stage are coupled to a unison ring that surrounds a casing of the compressor. The vanes are coupled to the unison ring such that rotation of the unison ring circumferentially about the casing causes a local rotation of the vanes attached thereto. Different techniques may be used to rotate the unison ring. Some techniques employ a torque tube coupled to the unison rings associated with the different stator stages for the compressor by a lever. In such techniques, however, actuation of the torque tube causes all of the unison rings coupled thereto to move in synchrony for a uniform distance and at a constant velocity. Therefore, the vanes of the different stator also rotate a proportional amount, but at a constant velocity in response to a particular amount of rotation of the torque tube.